http/app: iwyu

While working on tests, I found that |app.hpp| is missing some boost
headers. I added them manually in this commit.

Tested: code compiles.

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: I1d2fb0f312e1810d836c986e320263a9581f13f2
diff --git a/http/app.hpp b/http/app.hpp
index 59aca39..18147e4 100644
--- a/http/app.hpp
+++ b/http/app.hpp
@@ -8,6 +8,13 @@
 #include "routing.hpp"
 #include "utility.hpp"
 
+#include <boost/asio/io_context.hpp>
+#include <boost/asio/ip/tcp.hpp>
+#ifdef BMCWEB_ENABLE_SSL
+#include <boost/asio/ssl/context.hpp>
+#include <boost/beast/ssl/ssl_stream.hpp>
+#endif
+
 #include <chrono>
 #include <cstdint>
 #include <functional>